<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        .box1 {
            margin: 0;
            padding: 5px;
            height: 200px;
            background-color: #ddd;
            position: relative;
        }
        button {
            margin: 5px;
        }
        .box2 {
            width: 100px;
            height: 100px;
            background-color: red;
            position: absolute;
            left: 0;
        }
    </style>
</head>
<body>
<div class="box1">
    <button>运动到200</button>
    <button>运动到400</button>
    <div class="box2"></div>
</div>


<script>
	var btnArr = document.getElementsByTagName("button");
	var box2 = document.getElementsByClassName("box2")[0];

	var timer = null;
	
	btnArr[0].onclick = function(){
		animate(200);
	}
	btnArr[1].onclick = function(){
		animate(400);
	}
	
	function animate(target){
		clearInterval(timer);
		var speed = target>box2.offsetLeft?10:-10;
		timer = setInterval(function(){
			var val = target-box2.offsetLeft;
			box2.style.left = box2.offsetLeft + speed+ "px";
			if(Math.abs(val)<Math.abs(speed)){
				box2.style.left = target+"px";
				clearInterval(timer);
			}
		},30);
	}
</script>
</body>
</html>